Kryptonite is an ore from the planet Krypton which releases radiation. Depending on the color of the kryptonite, it has different effects. Most kryptonite only affects Kryptonians.
Kryptonite was first introduced in comics in 1949. It originated as a plot device from the Superman radio show. It's effect on Superman has become so commonly known that the word has entered the English language, defined as someone's weakness, similar to an Achilles heel or something that repulses. It can be slang for weaknesses as varied from crack cocaine to chocolate cake. "Kryptonite" has been licensed to a lock company.
Forms of Kryptonite
While Green Kryptonite is the most common form, Kryptonite has been discovered in various forms, generally differing in both appearance and effect.
The various forms of Kryptonite are
Black Kryptonite - In comics it is capable of making Superman a darker, evil being. But it first made an appearance in Smallville (the television series) where it is actually superheated Green Kryptonite that splits psyches of anyone into two physical manifestations: the normal mind and the darker, evil personality. This Kryptonite is different in the fact that it could effect both Kryptonians and humans (Clark and Lex). The similar kryptonite also appeared in Superman III movie. Although this version of green kryptonite was laced with tar. The tar-laced kryptonite acted like a red kryptonite that removed Superman's inhibitions. Later, Superman (Christopher Reeve) was split into two beings, an evil Superman and a good Clark Kent.
Blue Kryptonite - It is "Green Kryptonite" to Bizarro and in some cartoons and video games it heals Superman. In Smallville it appears in Episode 8 of Season 7 titled "Blue" and it makes Kal-El powerless as long as it is in contact with his skin, but on Krypton it is considered an honor to receive it as a ring on one's finger. It's effects are reversed on Clark's "Bizarro" twin, super charging him until he explodes in "Persona" (Season 7, Episode 10). Zod tries to use a blue kryptonite knife in "Salvation" (Season 9, Episode 21) to stay on Earth rather than be pulled to another dimension like his fellow Kryptonians, but takes the knife himself and Zod is sent away. In the New 52 reality it's said than blue kryptonite is the most lethal version of the mineral because induce spiritual death.
Pink Kryptonite - In Supergirl #79, when Linda Danvers takes the original Kara Zor-El's place and arrives in the Pre-Crisis era, there is a comical scene where Superman is apparently exposed to Pink Kryptonite, with the implication that it temporarily turned him gay.
Green Kryptonite - By far the most common variation. Green Kryptonite can weaken those of Kryptonian descent (weakening powers and abilities as well as strength), and in large enough doses or for long enough time it is lethal. In Smallville Green Kryptonite grants normal humans or other life mutations that make them super-powered and on most occasions psychotic. Here it also weakens Clark, at varying degrees including simply making him a little more human or leaving him on the brink of death. In New 52 appearances, the Kryptonite is again referred to as being rare, is shown that can be used as an energy source, to kryptonian rockets and possible to other vehicles. The mineral doesn't take away the Superman's powers, but is able to decrease it. It is unknown if has effects on humans.
Gold Kryptonite - It removes the abilities of Kryptonian for a maximum of fifteen seconds, though for a time there was an antidote that granted powers back for a short time.
Jewel Kryptonite - It comes from the Jewel Mountain range of Krypton. It magnifies the Phantom Zone resident's psychic powers, allowing them to project illusions into the "real-world" or to control the minds of non-Phantom Zone's denizens.
Orange Kryptonite - Gives normal cats Isis and Snooky super powers in Krypto the Superdog #4.
Red Kryptonite - Affects Superman differently every time for up to 3 days, as no piece of Red Kryptonite has the same effect but it can cause the oddest variety of effects including such things as transformations or even being unable to see anything green. In Smallville, Red Kryptonite removes Clark's inhibitions and cause him to act on all his impulses. In the New 52 DC, the red Kryptonite induce hallucinations and a lost in the grasp on reality.
Silver Kryptonite - Is harmless, made for Superman's "Silver Anniversary". Yet in Smallville it makes Superman paranoid when in his bloodstream, though in reality it was actually a hoax Kryptonite that was part of the BRAIN InterActive Construct-(Brainiac). Silver has made it's return found encased in the Ach-Om Rashay, the Amulet of Mindfulness. Superman was drawn to it and purposely exposed himself to it. It made him "highly unstable" as Batman put it. Superman immediately craved brownies and cereal. He viewed the others as if they were cartoons. The only cure was to seek out the other half which countered the effects. It is unknown how much was due to the Kryptonite or how much was due to a magical spell.
White Kryptonite - Destroys all plant life whether it is native to Krypton or not, created when Kryptonite passed through a weird space cloud.
X-Kryptonite - Gives non-powered lifeforms (Humans/ Animals) superpowers for a temporary duration. This is what gave Streaky the Supercat his powers, it was created by Supergirl who was seeking an antidote for Green Kryptonite.
Anti-Kryptonite - This peculiar form of Kryptonite exists in an alternate reality, where Ultraman (that reality's Superman) uses it as a power source. In fact, he gets weaker the farther away he is from it! But Pre-Crisis it is lethal to non-superpowered Kryptonians and killed many of them, it is their "Green Kryptonite".
Slow Kryptonite - is a variety of kryptonite produced by Metallo that affects humans. Why it is called "Slow Kryptonite" is because the rays sent out by normal Kryptonite are "fast" (high in frequency) and synced with the accelerated Kryptonian body. Slow Kryptonite, which releases "slow" rays, is synced with a slow human body, allowing it to affect it. Slow Kryptonite is the "Green Kryptonite" to normal humans, but its effect on actual Kryptonians is unknown.
Magno-Kryptonite - It was artificially created by Nero and it pulls in anything native to Krypton so that not even Bizarro or Superman can escape its pull.
Bizarro Red Kryptonite - It is the "Red Kryptonite" to normal humans.
Kryponite-X/Kryptisium - Professor Hamilton coined this term to refer to the change that occurred to the Kryptonite that the Cyborg Superman blasted the Eradicator with when it passed through the latter and struck Superman. Instead of being lethal, it restored Superman's powers. Unfortunately, it also apparently clung to him and caused him to absorb solar energy at a rapidly accelerated rate, eventually causing his powers to go out of control and his body to build more mass. Superman was purged of it after battling the Parasite. Not to be confused with X-Kryptonite.
There have also been hoax kryptonites such as Silver Kryptonite in Smallville (mentioned above), Yellow Kryptonite, Blood Kryptonite, Kryptonite Plus, Purple Spotted Kryptonite and Fake Kryptonite.
The Kryptonite Ring
Batman keeps a kryptonite ring inside a lead-lined compartment in his utility belt so that if Superman ever went rogue he would have a chance of controlling him. Lex Luthor once wore a kryptonite ring to protect himself from Superman. Prolonged exposure to it caused him to lose his hand.
In Smallville, most meteor rocks are shown for the first time or used in the form of rings, usually it is because people think of them as being beautiful rocks, however usually this causes harm to Clark who is unsuspecting of the rings.